This is a request for allthetropeswiki but I honestly don't see why it shouldn't be default for everyone.
https://www.mediawiki.org/wiki/Help:Extension:ParserFunctions#StringFunctions
All of these functions (len, pos, rpos, sub, replace, explode) are integrated from the StringFunctions extension, but are only available if an administrator sets $wgPFEnableStringFunctions = true; in LocalSettings.php.
All of these functions operate in O(n) time complexity, making them safe against DoS attacks.
Note we already have Extension:Regex Functions as an option -- these should be cheaper and easier not to screw up.
So, I propose adding this:
'wgPFEnableStringFunctions' => [ 'default' => true, ],